Adapt WebUI to use REST for suggesting branches for cherry-pick

Adapt the cherry-pick dialog to use the REST endpoint for listing
branches to suggest branch names to the user.

The old RPC to list branches is deleted since it is not used anymore.

Change-Id: I13a6894d4a5f1e3cdb6e8e2682992a742d7660a4
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
3 files changed